The present invention relates to a recording medium, a recording method, a reproducing method, an erasing method, a reproducing apparatus, and a recording.multidot.reproducing apparatus and, more particularly, to a recording medium, a recording method, a reproducing method, an erasing method, a reproducing apparatus, and a recording.multidot.reproducing apparatus in which the size of a region for recording information is the wavelength of light or smaller.
In the recent information society, the amount of information keeps on increasing. Therefore, the advent of a recording.multidot.reproducing method having a recording density extraordinarily higher than the conventional recording densities and a recording apparatus and a reproducing apparatus based on the method is being expected.
As techniques for realizing super-high density recording, a number of scanning probe techniques such as an STM, an AFM, and a Near-field Scanning Optical Microscope (NSOM) are being studied. In effect, these techniques are beginning to be applied to storage devices. In particular, an NSOM has attracted a great deal of attention as a means for realizing optical recording in which the bit size (the size of the unit recording area) is the wavelength of light or smaller.
Betzig et al. have tried Magneto-Optic (MO) information recording and reproduction by irradiating light from an Ar ion laser onto a Co/Pt multilayered film by using an NSOM probe and formed a recording pattern 60 nm in diameter (Appl. Phys. Lett. 61, 142(1992)).
Also, Hosaka et al. have realized phase change recording on a scale of a diameter of 50 nm by irradiating light from a semiconductor laser onto a 30-nm thick Ge.sub.2 Sb.sub.2 Te.sub.5 film by using an NSOM probe (Thin Solid Films 273, 122(1996), J. Appl. Phys. 79, 8082(1996)).
These researches have proven that recording on a scale of the wavelength of light or smaller is possible by the application of an NSOM to a recording.multidot.reproducing method using MO or phase change which are presently used in optical recording. However, these methods have several problems.
First, bit size control is very difficult to perform, regardless of which of MO or phase change is used, because it is necessary to heat to a predetermined temperature by using absorption of a laser beam when information is recorded.
To decrease the bit size, an NSOM probe with a small aperture diameter must be used so that the spot size of light decreases. However, it is very difficult to manufacture an NSOM probe with a small aperture diameter.
Also, even if it is possible to manufacture an NSOM probe with a small aperture diameter, the intensity of light irradiated on a recording medium lowers as the aperture diameter of the NSOM probe is decreased. In the methods described above, high-power light must be irradiated in recording. For this purpose, light having higher power must be sent to the optical fiber of the NSOM probe. Consequently, the end portion of the NSOM probe is heated to a considerably high temperature, and this can lead to peeling of the metal covering the end portion.
Furthermore, even if the spot size is thus decreased, the bit size is increased by the effect of heat diffusion in the recording medium. This also makes the bit size difficult to control.
Second, the reproduction signal is weakened as the bit size is decreased, and this makes detection difficult. For example, the Kerr rotation angle is small in the recording.multidot.reproducing method using MO. Therefore, if the bit size is decreased, a weak reproduction signal becomes difficult to detect.
In the recording.multidot.reproducing method using phase change, as the bit size is decreased, the difference in reflectance between the recording area and non-recording area decreases. According to the results of experiments by Hosaka et al. described above, even when the bit size is about a few tens of nm, the intensity difference between a reproduction signal corresponding to information "0" and a reproduction signal corresponding to information "1", i.e., the reflectance difference decreases at a ratio larger than the reduction ratio of the bit size. This makes detection (identification) of reproduction signals difficult. Although no experimental result is obtained about the reflectance difference when the bit size is further decreased, it is readily predicted that the difference further decreases.
Increasing the recording density to a super high density of terabit/cm.sup.2 or more amounts to decreasing the size of the unit recording area to 10 nm or less. It is considered that such super-high density recording using NSOM techniques is possible in principle. As described above, however, the various technical problems arise when the conventional recording media are used to realize this super-high density recording. Accordingly, the advent of an entirely novel recording.multidot.reproducing technique effective when the size of the unit recording area is on a nanometer scale is being expected.